1. Field of the Invention
This invention is directed to safety circuits for use with electrosurgical apparatus and to a method of using such circuitry.
2. Discussion of the Prior Art
Various safety circuits have been described in the prior art for use with electrosurgical systems. For example, in U.S. Pat. No. 3,683,923, granted to Robert K. Anderson, circuitry for detecting, inter alia, a discontinuity in the patient circuit is described where the patient circuit includes the patient or indifferent electrode and the patient lead connected thereto. Such a discontinuity can occur, for example, whenever the patient loses contact with the patient electrode or a break occurs in the patient lead. The current from the active electrode will seek ground through an alternate current return path whenever a patient circuit discontinuity occurs. Such a path may be provided through a monitoring electrode attached to the patient's body or any other grounded object in inadvertent contact therewith. Since a monitoring electrode or the like may contact the patient over a very small area, the current density thereat will be quite high thereby tending to burn the patient. Further, since the monitoring electrode or the like may be covered by a sheet, the burning may occur for a substantial period of time before it is detected. Hence, it is very important that appropriate means be provided to detect a discontinuity in the patient circuit and take appropriate action upon the detection thereof such as the sounding of an alarm and/or the de-energization of the generator.
The aforesaid U.S. Pat. No. 3,683,923, which is incorporated herein by reference, detects a discontinuity in the patient circuit by taking the difference between the radio frequency current in the active lead and that in the patient lead. Whenever this difference exceeds a predetermined threshold, an alarm is sounded. In particular, the alarm is sounded whenever the active lead current exceeds the patient lead current by the threshold value or vice versa -- that is, whenever the patient lead current exceeds the active lead current. The foregoing results from the fact that the difference is taken between the radio frequency currents in the active and patient leads. However, the dangerous condition corresponding to a patient circuit discontinuity occurs when the active lead current exceeds the patient lead current. In fact, the radio frequency patient lead current can exceed the radio frequency active lead current without danger to the patient. Thus, if two electrosurgical generators with two respective instruments are in contact with a patient's body as is sometimes the case and one of the instruments is active and the other is idle, the current from the active instrument will divide between the patient leads for the two generators whereby the patient current for the idle generator will exceed that in the active lead thereof. With a safety circuit as disclosed in the aforementioned Anderson patent or the aforementioned Newton patent application, the aforesaid condition would generate a nuisance alarm -- that is, an alarm when the patient is not in danger.
It is thus a primary purpose of this invention to provide an improved safety circuit for use with an electrosurgical generator which is sensitive only to an excess of active lead current over patient lead current to thereby detect a discontinuity in the patient lead circuit without generating nuisance alarms whenever the patient lead current exceeds that of the active lead.
In the aforementioned application Ser. No. 520,269, a current sensing device is employed in the grounding connection of the patient lead where, if a predetermined current is sensed, an alarm condition is signalled. However, whenever coagulation waveforms of high crest factor (defined as the peak vaue in voltage or current divided by the rms value) are employed, the magnitude and higher harmonic components of the generator output signal will increase thereby increasing the active lead leakage current through the stray capacitance from the active lead to ground. This current will also be sensed by the current sensor and tend to generate a nuisance alarm since the current is not dangerous to the patient.
It is thus a further object of this invention to provide a safety circuit of the foregoing type wherein a dynamically variable threshold is employed to thereby permit the lowest possible threshold for sounding an alarm without the production of nuisance alarms associated with the use of high performance waveforms such as coagulation waveforms having high peak to peak voltages and crest factors.
As brought out in the aforesaid patent application Ser. No. 520,269, it is desirable to substantially ground the patient lead for radio frequency currents to thereby prevent the elevation of the patient to an undesirable potential. Thus, anesthesiologists quite often like to touch the patient during a surgical procedure to monitor temperature, pulse rate and the like. Since the area of contact with the patient, due to a light touch, is quite small, the person touching the patient will experience a slight burning sensation if the patient is elevated to an undue potential. This burning sensation although typically not dangerous is often disconcerting. If the patient lead is isolated from ground at radio frequencies, the abovementioned leakage currents from the active lead to ground can raise the potential of the patient lead with respect to ground due to the coupling of this current through stray capacitance from the patient lead to ground. The active lead to ground leakage current, as stated above, increases with increasing crest factor, for example. Hence, whenever a high performance level is desired, it is difficult to use an isolated output circuit since the patient lead potential rises to an undesirable level and hence the patient lead msut be substantially grounded at radio frequencies. Further, if the active electrode of an isolated output circuit accidentally contacts a grounded object, the patient would be raised to the full output potential of the generator if the patient were not kept at about ground potential for radio frequency current. Thus, this is a further reason for grounding the patient lead at radio frequencies.
However, grounding of the patient lead is undesirable at low frequencies such as the 60 Hz power frequency. That is, if the patient comes in contact with, for example, a faulty piece of equipment which is at power frequency potential, this potential could be conducted through the patient to the grounded patient lead thereby subjecting the patient to the extreme danger of electrocution. Hence, at power frequencies and the like it is desirable to effectively isolate the patient lead from the ground and thus not establish a path where high, power frequency voltages can pass through a patient upon inadvertent contact with a faulty piece of equipment. In the aforementioned Newton patent application, the foregoing is effected by providing a frequency sensitive network between the patient lead and ground whereby a low impedance is presented to radio frequency currents and a high impedance is presented to power frequency currents and the like. This can be achieved by the appropriate selection of a capacitor, the value of which is low enough to present a high impedance at the low frequencies and high enough to present a low impedance at the radio frequencies where radio frequencies for the purposes of this application extend approximately from 200 kilohertz to 5 megahertz although there is no intent to be limited to this particular range. A limitation on how small a capacitor may be employed is that of the voltage on the patient lead. As stated above, this voltage must not raise the potential of the patient too high. As also stated above, when using electrosurgical generators operating at a high level of performance, high voltage levels are typically employed. Thus, there is a tendency for the voltage across the capacitor to increase with these higher voltages levels.
Hence, it is a further object of this invention to provide circuitry for decreasing the effective voltage across the capacitor in a frequency sensitive network of the type described above whereby a smaller capacitor may be used while, at the same time, decreasing the voltage thereacross.
These and other objects of the invention will become apparent from a reading of the following specification and claims taken together with the drawing.